The isotopic analysis of the d13C and d15N in fossil plants have been applied in different basins around the world seeking for diagenetic signals; stable isotopic composition of atmospheric CO2; vegetation types (i.e. C3 and C4 species); and local paleoenvironmental parameters. In this contribution is analyzed the Upper Triassic Ischigualasto Formation flora, dated as Carnian on the base of two radiometric ages (227,8„b0,3 on the base and 218„b1,7 on the top). This Formation yields a very well preserved fossil flora that included roots, tree-trunks, palinomorphs and leaves, mainly of Corystospermaceae. Most of the fossil flora of the Ischigualasto Formation is concentrated along paleochannels and marsh deposits in the upper portion of the formation. The stable isotopic composition (d13C and d15N) and carbon/nitrogen (C/N) ratios have been measured in 160 samples. Along Ischigualasto flora assemblage no correspondence was observed between the C/N ratios and d15N and d13C values indicating that diagenesis has not significantly affected the Formation isotopic signal of the floras. The d13C isotopic values showed a mean d13C of -25.27, ranging between -26.91 and -22.65, these values correspond to average C3 plants. However these values are higher than the mean value of extant C3 plants living in an environment without any significant stress (-28.0o/∞). This value could be explained by water/salinity stress under an extremely seasonal climate suggested based on sedimentological, taphonomical and pedogenetical signals. Also results in a CO2 atmospheric isotopic composition of -6 o/∞, which is close enough to the expected for the Late Triassic (-6.1 o/∞ ± 0.5 o/∞). Startling, one of the cuticle analyzed produce an unexpected value of -15.69, which is consistent with the values of C4 plants (considered Miocene). Also, an intramarsh variation in carbon isotopic values was found as the result of variation in water/salinity stress of local ecological parameters.